1. Field Of The Invention
The present invention relates generally to cabling machines, or stranders, and more particularly to apparatus and a method for winding a plurality of conductor strands on a core strand to form a self-neutralized multi-strand cable, each strand being continuously free of any backtwist. The present invention finds particular utility in applications for winding small gauge or fragile strands, such as fiber optic strands. Of course, the present invention may be used with equal advantage in many other applications, such as insulated wire, steel wire, or other products requiring stranding without torsional twisting.
2. Description Of The Prior Art
Stranded cables are well known and have been used in many applications for more than a century. For example, stranded wire cables are used for structural applications, such as bridges, for electrical applications, such as electrical transmission lines, and for communications applications, such as telecommunication transmission cables. Recently, stranded fiber optic cables also have been used for various applications, including telecommunication transmission lines.
Stranders also are well known. Generally, a strander winds together a plurality of strands to form a flexible multi-strand cable. In some cases, a plurality of strands, or conductors, are wound around a center or core strand. In other applications, individual conductor strands simply are wound about each other. The individual strands typically are fed to the strander from respective reels, or bobbins.
Known stranders generally are classified in one of two categories; rigid stranders and planetary stranders. For example, U.S. Pat. Nos. 3,396,522 (Biagini) and 3,727,390 (Schwarz) disclose rigid stranders that include a plurality of reels disposed radially on a disk-shaped platform. The disk-shaped platform is mounted on a cradle and arranged for rotation about a core strand, which is drawn therethrough on a common axis. In such rigid stranders, conductor strands are unwound or payed-off from individual reels, fed along the length of the cradle to a closing device, and layed on the core strand as it is drawn through the common axis of the disk-shaped platform and strander cradle. The windings of the stranded cable are generated by rotating the disk-shaped platform about the common axis as the core strand is drawn therethrough.
Other examples of stranders include tubular stranders and bow stranders. In a tubular strander, a bunch of strands are fed through a long pipe, which is turned to twist the strands. A bow strander comprises a plurality of bow members having their ends arranged on a common axis and their bodies bowed radially outward. In a bow strander, the strands are payed-off reels which are held in cradles, and the bows are arranged to allow the strands to skip over the reels for passage through a hub.
Although rigid stranders provide utility in a number of applications, they suffer a number of drawbacks. Bow stranders generally manipulate the strands extensively through small pulleys and are limited in the number of strands that can be twisted at one time. Moreover, all rigid stranders have a drawback in that they add a twist to each strand (known as a backtwist), as the strand is wound about the other strands to form the resultant cable. This backtwist stretches the strand and degrades the tensile and shear strength of the strand, as well as the resultant cable. For small or fragile strands, such as fiber optic strands, this degradation may cause the strands or cable to fail or break. Moreover, even if the fiber optic strands do not break, a backtwist may degrade the optical transmission efficiency of a fiber optic strand because twisting alters the physical continuity of the fiber walls, and causes clouding of the transmitted signal.
Planetary stranders provide a multi-strand cable in which the individual conductor strands in the cable do not have a backtwist. For example, U.S. Pat. Nos. 2,802,328 (Ritchie), 3,010,275 (Khartmann) and 3,058,867 (Plummer) disclose planetary stranders similar to the above-described rigid stranders, wherein individual conductor strands are fed from respective reels disposed radially on a disk-shaped platform, and wherein the disk-shaped platform is mounted on a cradle for rotation about the common axis of a core strand. However, in such planetary stranders, each reel is rotatably supported on the disk-shaped platform about an axis parallel to the common axis, such that for each revolution of the platform about the common axis, each reel also rotates one revolution about its parallel axis. In this manner, the strands are payed-off from the reels, fed along the length of the cradle to a closing device, and layed on the core strand to form a twist-neutralized multi-strand cable.
Although planetary stranders have utility in many applications, known planetary stranders also have certain drawbacks. One drawback is bowing of the conductor strands during the reel unwinding operation. That is, the conductor strands are payed-off from the reels and fed along the length of the strander cradle to a closing device, centrifugal force acts on the payed-out conductor strands. These forces cause the conductor strands to bow radially outward. At low rotational speeds, this effect may be inconsequential. However, radial bowing increases as the rotational speed and radial distance from the axis of rotation increases.
Bowing also may be caused by windage. Rotation of the conductor strands at a radius about the axis of rotation creates windage for the conductor strands. Windage forces cause the strands to bow in a direction opposite the direction of rotation. Thus, bowing due to windage also increases as the rotational speed or radius increases.
It is known to reduce bowing by increasing the tension in the conductor strands. However, increasing the tension increases the likelihood of breakage or degradation of the conductor strands or cable. This is particularly true for fiber optic strands, which are fragile and subject to degradation of transmission efficiency when stretched or bent through an angle beyond their minimum bend radius. In addition, in order to maintain a quality stranded cable having consistent winding characteristics, the tension in the strands must be controlled so that it remains substantially constant. Since tension from centrifugal and windage forces in known stranders varies directly with the rotational speed and distance from the axis of rotation, these stranders generally require complex tension control devices for maintaining a constant tension over a range of rotational speeds. This is particularly true for optical fibers, which generally have a limited acceptable tension range.
It also is known to decrease bowing due to windage by feeding the conductor strands through respective guides or pipes disposed between the reels and the closing device. However, this arrangement increases the weight and complexity of the strander. Also, the energy required for rotation increases because guides and pipes are subject to windage and to radial acceleration forces caused by the centrifugal force. Moreover, the strands continue to bow radially due to the centrifugal force, and thus are subject to degradation from friction and wear of the conductor strands against the guides or pipes.
The above described drawbacks of known planetary stranders undesirably limit production speed for twist neutralized multi-strand cable. Specifically, there is a trade-off between rotational speed (thus production speed) and product quality. That is, for example, a fiber optic strand will develop a "history" with each bend to which it is subjected, and the quality of the strand will degrade with each such bend. Known planetary stranders generally are limited to operational speeds in the range of 75 to 150 RPM, with a maximum speed of about 200 RPM.